Current Research Projects

Neogene Tectonics of Southern California

The focus of this research project is to investigate the timing of rotation of the Transverse Ranges and
the evolution of the 3-D architecture of the Los Angeles basin. Objectives are to understand better the seismicity
of the region and the relationships between petroleum accumulations and the structure and stratigraphic evolution
of the basin. Figure 1 shows the main physiographic and structural features of the Los Angeles basin region, the
epicenter of recent significant earthquakes and the our initial study area in the northeastern Los Angeles basin.

Los Angeles basin tectonic model:

Most tectonic models attribute the opening of the Los Angeles basin to lithospheric extension produced by breakaway
of the Western Transverse Ranges from the Peninsular Ranges and 90 degrees or more of clockwise rotation from
ca. 18 Ma to the present. Evidence of this extension includes crustal thinning on tomographic profiles between
the Santa Ana Mountains and the Santa Monica Mountains and the presence in the Los Angeles basin of Middle
Miocene volcanic rocks and proto-normal faults. Detailed evidence of the 3-D architecture of the rift created
by the breakaway and the timing of the rift phase has remained elusive.

The closing of the Los Angeles basin in response to N-S contraction began at ca. 8 Ma and continues
today (Bjorklund, et al., 2002). A system of active faults has developed that pose significant seismic hazards for the
greater Los Angeles region. Crustal heterogeneities that developed during the extension phase of basin development may
have strongly influenced the location of these faults. Uncertainties in fault location geometries and in how movement
on one fault may affect movement on other faults have not yet been well quantified. We plan to carry out regionally
integrated analyses of geophysical and geologic data that we hope will lead to improved 3-D models of the Los Angeles
basin and better definition of the seismic hazards.

Whittier fault model:

Whittier fault is a high-angle reverse fault, which trends N70oW along a 40 km strike-length, which
is part of the San Andreas fault system and the subject of our initial study in the northeastern Los Angeles basin
(Bjorklund and Burke, 2002). Figure 2 is a cross section located in the central part of the Whittier fault trend.
Along the central part of the structure, strata in the hanging wall block form a broad anticline and strata in the
footwall block dip steeply south on the north limb of an asymmetric syncline. The maximum vertical separation due
to folding and offset along the Whittier fault is more than 4 km. Analysis of surface geologic maps and data from
hundreds of oil wells and the application of balanced cross section techniques reveal a three-phase evolution of the
Whittier fault that we have represented graphically in Figure 3. The three phases consist of the following.

Rotation of the Western Transverse Ranges block and the formation of the proto-Whittier normal fault and
associated half-graben (ca. 14-10 Ma). During that time, organic-rich shale (oil source rock) was deposited in
the half-graben and throughout the Los Angeles basin.

Transpression along the San Andreas fault and reactivation of the proto-Whittier fault as a basement-involved
reverse fault (ca. 8 Ma). Over the next 8 million years, structural uplift continued and turbidite fan-channel
systems and pelagic sediments eventually filled the Los Angeles basin. Beginning at ca. 3 Ma, oil generated from
the organic-rich shale accumulated in structures along the Whittier fault, which became the oil fields that exist
there today.

Breakthrough onto the seafloor of the reactivated proto-Whittier fault and formation of the present throughgoing
Whittier fault system (ca. 0.5 Ma). Continued uplift led to subaerial exposure of the hanging wall block.

Over 600 MMBO have been produced from structural and stratigraphic traps along the Whittier fault
and about 35 MMBO remain to be produced. Oil accumulations range in depth from less than 200 meters to 2400 meters
and include simple structural closures, faulted noses, combination structural and stratigraphic traps and purely
stratigraphic traps, one of which is a large accumulation in the footwall syncline of the inverted Whittier fault
structure (Yorba Linda field). During the period of structural inversion on the Whittier fault, an oil accumulation,
which would have become the largest oil field in the Los Angeles basin, may have formed in the Soquel sandstone on
the crest of the inverted structure Recoverable oil could have exceeded 3 billion barrels. If the accumulation
existed, it has since been uplifted and completely eroded, leaving to be produced today small, fractured reservoirs
on the crest of the exhumed structure and downdip accumulations on the flanks of the structure.

Hydrocarbon accumulations associated with inversion structures like the Whittier fault may lie
buried in the undrilled deeper parts of the Los Angeles basin. Such structures may not have experienced the complete
cycle of deformation exhibited by the Whittier fault and may now exist as simple half grabens or as inverted half
grabens with folded but unfaulted sedimentary strata. The first possibility could juxtapose Miocene source rocks and
Paleogene and Cretaceous sandstones that thus far have been unproductive and largely undrilled in the Los Angeles
basin. The second possibility could result in large, low relief structures with the potential for significant
hydrocarbon accumulations that might be difficult to recognize on vintage seismic data.

Future Research

Our future research in the Los Angeles basin will expand the studies begun in the northeastern
Los Angeles basin to the entire basin. The studies will investigate the merits of applying the Whittier fault
model to other important faults in the basin to understand better the seismicity and structural evolution of the
basin. We also hope that the results of the studies will allow us to quantify the remaining potential for
significant undiscovered hydrocarbon accumulations in the basin. The cross sections in Figure 4 are representative
of cross sections that we plan to construct throughout the Los Angeles basin (Bjorklund, et al., 2002).

Figure 1. Index map of the Los Angeles basin region. The red-dashed rectangle in the center
of the map locates the Whittier fault and our northeastern Los Angeles basin study area. Lines A-A', B-B' and
C-C' are the locations of cross sections in Figures 2, 3 and 4. The red stars are the epicenters of important
recent earthquakes in the area.

Figure 2. Miocene turbidite fan-channel sandstones are productive in the footwall block block
of the Whittier fault on the steeply-dipping south flank of the Puente Hills anticline. Stratigraphically
controlled oil accumulations in Pliocene rocks are present in the footwall syncline east and west of the cross
section. Over 3 MMBO have been produced since 1880 from fractured Upper Miocene La Vida shale, Middle Miocene
Topanga sandstone and Jurassic basement rocks on the crest of the Puente Hills anticline.

Figure 4. Cross sections B-B' and C-C' combine well and outcrop data and tomographic
velocities to show spatial relationships of volcanic rocks (red) and inferred, Miocene plutons (black) that are
the possible sources of volcanic sills and flows (red). Dashed rectangles are velocity model grid blocks with
average P-wave velocities (km/s) from Hua-wei Zhou’s Southern California velocity model. We interpret the
higher-velocity anomalies to indicate a vertical, stock-like pluton (cross section C-C') and a tabular, sill-like
pluton (cross section B-B') that we have named the Whittier Narrows and El Modeno plutons, respectively. We
suggest that both plutons were emplaced into the upper crust during the Miocene and were the magma sources for
the overlying volcanic rocks. We projected the hypocenter of the recent Chino Hills earthquake (July 29, 2008)
(location from Hauksson, et al.) to the cross sections at the depth of the earthquake. The location of the
Whittier fault on cross section C-C' is much shallower than the depth of the projected hypocenter because the
fault dips 60 degrees to the north and the hypocenter is located north of the cross section line. The western
part of the section is parallel to the strike of the Whittier fault. The Chino Hills earthquake and its
aftershocks (not shown) do not appear to relate simply to a single known fault, although the focal mechanism
of the initial event is consistent with our interpretation of the geometry and kinematics of the Whittier fault.

University of HoustonCollege of Natural Sciences and MathematicsScience & Research Building 13507 Cullen Blvd, Room 214Houston, Texas 77204-5008713-743-2611